Xylene isomers find wide and varied application. They are especially valuable as intermediates in chemical processes. By way of example, para-xylene (PX) is a feedstock for terephthalic acid, which finds use in the manufacture of polyester fibers and films; meta-xylene (MX) is used in the manufacture of dyes; and ortho-xylene (OX) is used as a feedstock for phthalic anhydride, which finds use in the manufacture of plasticizers. PX is currently the most valuable of the xylene isomers and, although research related to obtaining (e.g., producing or purifying) PX is too voluminous to mention, there is still intensive research in the area.
There are many possible feeds currently used to obtain PX. The majority of para-xylene produced today comes from catalytic reforming, which involves dehydrogenation and dehydrocyclization of naphtha feedstocks. The effluent of the reforming process, known as reformate, is rich in aromatics, particularly benzene, toluene, and mixed xylenes (BTX), and is used as feedstock to aromatics plants. Processes exist to increase the yield of para-xylene over the equilibrium mixture in the reformate, including selective toluene, disproportionation and selective methylation of benzene and/or toluene with methanol.
Recently, significant research has focused on finding alternative sources and methods for producing BTX and particularly para-xylene. For example, although steam cracking, or pyrolysis, is the preferred method of producing light olefins (ethylene, propylene, and butenes) from heavier hydrocarbon feedstocks, the process also generates a by-product termed pyrolysis gasoline, steam cracked naphtha (SCN) or pygas. Pygas is a complex mixture of C6 to C10+ hydrocarbons that is rich in aromatics, particularly benzene and toluene, but also contains C8, C9, and C10+ aromatics. Similarly, catalytic cracking, particularly fluid catalytic cracking (FCC), in addition to producing fuels and light olefins, generates a C6 to C10 aromatic rich stream which is similar to pygas and is generally known as cat naphtha. There is significant interest in developing methods of upgrading refinery sources, such as pygas and cat naphtha, to increase the yield of BTX and preferably para-xylene.
For example, U.S. Pat. No. 6,635,792 discloses a process for producing BTX and liquefied petroleum gas (LPG) from a hydrocarbon feedstock having boiling points of 30° C. to 250° C., such as reformate and pyrolysis gasoline. In the process, aromatic components in the hydrocarbon feedstock are converted to BTX-enriched components in the liquid phase through hydrodealkylation and/or transalkylation, and non-aromatic components are converted to LPG-enriched gaseous materials through hydrocracking. The process employs a catalyst comprising platinum/tin or platinum/lead on mordenite, zeolite beta or ZSM-5. U.S. Pat. Nos. 7,297,831 and 7,301,063 disclose similar processes.
U.S. Pat. No. 7,563,358 discloses a process for producing BTX-enriched product from a hydrocarbon feed comprising: (a) C6+ non-aromatic cyclic hydrocarbons; (b) C8+ single-ring aromatic hydrocarbons having at least one alkyl group containing two or more carbon atoms; and (c) C9+ single-ring aromatic hydrocarbons having at least three methyl groups, by contacting the feed in the presence of hydrogen with a catalyst comprising at least one Group VIII metal and a large or intermediate pore molecular sieve having an alpha value, before incorporation of the Group VIII metal, from about 2 to less than 100 under conditions sufficient for (i) forming aromatic hydrocarbons from C6+ non-aromatic cyclic hydrocarbons; (ii) dealkylating C8+ single-ring aromatic hydrocarbons having at least one alkyl group containing two or more carbon atoms; (iii) transalkylating C9+ single-ring aromatic hydrocarbons having at least three methyl groups; and (iv) disproportionating toluene, to produce a product containing an increased amount of BTX compared to the feed. A preferred hydrocarbon feed is steam cracked naphtha.
Although steam cracked naphtha is an excellent source of BTX in the refinery, it will be understood the feedstock for a conventional steam cracking unit must be subjected to substantial (and expensive) processing before it reaches the unit. Normally, whole crude is first subjected to desalting prior to being distilled or otherwise fractionated or cracked into a plurality of fractions, such as gasoline, kerosene, naphtha, gas oil (vacuum or atmospheric), and the like, including a high boiling residuum (“resid”). The resid cut typically has a boiling point of greater than about 650° F. (343° C.), at about atmospheric pressure. After desalting and removal of the resid fractions, any of the remaining fractions other than the 650° F.+(343° C.+) resid, may be passed to a steam cracker as the feedstock for that plant. Such resid fractions are, however, also potential sources of BTX and/or olefins although, in view of their highly viscous nature, they normally require expensive pretreatment by hydrotreating and/or visbreaking before they can be processed.
For example, U.S. Pat. No. 7,972,498 discloses a process for producing olefins comprising: (i) hydroprocessing a feed comprising crude comprising resid or a crude fraction comprising resid in a hydroprocessing unit at a temperature sufficient to promote incipient thermal cracking of the resid, wherein the resid includes 1050° F.+(565° C.+) resid; (ii) obtaining hydrogenated C2+ effluent from a resid hydroprocessing unit, wherein the effluent comprises resid; (iii) separating the effluent in a separator that comprises at least one of a visbreaker, a flash drum, a high pressure separator, and a vapor liquid separator wherein the effluent in the separator is heated at visbreaking conditions to a temperature of at least about 850° F. (454° C.), into an overhead stream and a bottoms stream, the bottoms stream comprising hydroprocessed resid including 1050° F.+(565° C.+) resid; (iv) feeding the overhead stream as vapor to a steam cracker; (v) steam cracking the overhead stream and obtaining a steam cracker effluent from the steam cracker comprising olefins; (vi) feeding the bottoms stream to a catalytic cracking unit; and (vii) cracking the bottoms stream in the catalytic cracking unit to obtain a catalytically cracked stream comprising at least one of gas oil and olefins; and further comprising flashing the effluent through at least one pressure drop to reduce the pressure of the effluent, prior to or during the step of separating the effluent. U.S. Pat. No. 4,257,871 discloses a process for producing olefins from a vacuum residue by first separating, preferably by solvent extraction, the asphalt therein, blending resultant asphalt-depleted fraction with a lighter fraction, e.g., a vacuum gas oil, and then subjecting the blend to a conventional catalytic hydrogenation step prior to thermal cracking.
In addition, U.S. Published Patent Application No. 2005/0209495 discloses process for steam cracking a heavy hydrocarbon feedstock, said process comprising: (a) heating a heavy hydrocarbon feedstock; (b) mixing the heavy hydrocarbon feedstock with a fluid to form a mixture stream; (c) flashing the mixture stream to form a vapor phase and a liquid phase; (d) removing the liquid phase in a flash/separation vessel; (e) cracking the vapor phase in a radiant section of a pyrolysis furnace to produce an effluent comprising olefins, said pyrolysis furnace comprising a radiant section and a convection section; and (f) quenching the effluent using a transfer line exchanger, wherein the amount of the fluid mixed with the heavy hydrocarbon feedstock is varied in accordance with at least one selected operating parameter of the process.
Additional references of interest include: U.S. Pat. No. 7,176,339.
According to the present invention, it has now been found that by blending resid fractions with pyrolysis gasoline and/or similar aromatic refinery streams, the viscosity of the blend can be reduced sufficiently to allow the blend to be fed to a catalytic pyrolysis unit to produce an effluent stream with an increased benzene and/or toluene content and a C3− olefin by-product.